Recently as our society is increasingly information-oriented, communications of information are becoming increasingly important, and eavesdropping and improper use of information are becoming an increasingly serious concern. To prevent eavesdropping, information has conventionally been encrypted and transmitted.
Information is encrypted and communicated between terminals in systems. Among them, there are public key cryptography and private key cryptography. Public key cryptography provides a high level of safety, however it is not suitable for encrypting large amounts of data.
In contrast, private key cryptography allows a process to be relatively readily performed and also allows large amounts of data to be rapidly encrypted. Private key cryptography, however, requires transmitting a private key to the counterpart of the communication. Furthermore in private key cryptography if the same private key is continuously used it becomes pervious to decryption and the system's safety may be damaged.
Accordingly, as a method sharing a private key without transmitting the key to a counterpart is suggested measuring a characteristic of a transmission path between two terminals and using the measured characteristic to generate a private key at each terminal (Motoki Horiike, Hideichi Sasaoka, “A Scheme of Secret Key Agreement Based on the Random Fluctuation of Channel Characteristics in Land Mobile Radio”, The Institute of Electronics, Information and Communication Engineers, October 2002, TECHNICAL REPORT OF IEICE RCS2002-173, pp. 7-12).
In this method, a delay profile provided when data is communicated between two terminals is measured at each terminal and converted from an analog signal to a digital signal to generate a private key at each terminal. More specifically, an radio wave propagating through a transmission path exhibits reversibility, and a delay profile provided when data is transmitted from one terminal to the other terminal is identical to that provided when the same data is transmitted from the other terminal to one terminal. Accordingly, a private key generated as based on the delay profile measured at one terminal is identical to that generated as based on the delay profile measured at the other terminal.
Thus, in the method utilizing a transmission path's characteristic to generate a private key, simply communicating single data between two terminals enables a single private key to be shared.
If the data transmitted between the two terminals is eavesdropped by a third party in the vicinity of each terminal and the delay profile is measured, however, the eavesdropper can obtain a delay profile close to that measured at each terminal and may succeed in decrypting the private key.
The present invention therefore contemplates a radio communications system capable of reducing eavesdropping of private key.